Regulation and function of the mammalian central nervous system is governed by a series of interdependent receptors, neurons, neurotransmitters, and proteins. The neurons play a vital role in this system, for when externally or internally stimulated, they react by releasing neurotransmitters that bind to specific proteins. Common examples of endogenous small molecule neurotransmitters such as acetylcholine, adrenaline, norepinephrine, dopamine, serotonin, glutamate, and gamma-aminobutyric acid are well known, as are the specific receptors that recognize these compounds as ligands (“The Biochemical Basis of Neuropharmacology”, Sixth Edition, Cooper, J. R.; Bloom, F. E.; Roth, R. H. Eds., Oxford University Press, New York, N.Y. 1991).
In addition to the endogenous small molecule neurotransmitters, there is increasing evidence that neuropeptides play an integral role in neuronal operations. Neuropeptides are now believed to be co-localized with perhaps more than one-half of the 100 billion neurons of the human central nervous system. In addition to being found in humans, neuropeptides have been discovered in a number of animal species. In some instances, the composition of these peptides is remarkably homogenous among species. This finding suggests that the function of neuropeptides is vital and has been impervious to evolutionary changes. Furthermore, neuropeptides, unlike small molecule neurotransmitters, are typically synthesized by the neuronal ribosome. In some cases, the active neuropeptides are produced as part of a larger protein that is enzymatically processed to yield the active substance. Based upon these differences, compared to small molecule neurotransmitters, neuropeptide-based strategies may offer novel therapies for the treatment of CNS diseases and disorders. Specifically, agents that affect the binding of neuropeptides to their respective receptors or ameliorate responses that are mediated by neuropeptides are potential therapies for diseases associated with neuropeptides.
There are a number of afflictions that are associated with the complex interdependent system of receptors and ligands within the central nervous system; these include neurodegenerative diseases, affective disorders such as anxiety, depression, pain and schizophrenia, and affective conditions that include a metabolic component, namely obesity. Such conditions, disorders, and diseases have been treated with small molecules and peptides that modulate neuronal responses to endogenous neurotransmitters.
One example of this class of neuropeptides is neuropeptide Y (NPY). NPY was first isolated from porcine brain (Tatemoto, K. et al. Nature 1982, 296, 659) and was shown to be structurally similar to other members of the pancreatic polypeptide (PP) family such as peptide YY, which is primarily synthesized by endocrine cells in the gut, and pancreatic polypeptide, which is synthesized by the pancreas. NPY is a single peptide protein that consists of thirty-six amino acids containing an amidated C-terminus. Like other members of the pancreatic polypeptide family, NPY has a distinctive conformation that consists of an N-terminal polyproline helical region and an amphiphilic alpha-helix joined by a characteristic PP-fold (Vladimir, S. et al. Biochemistry 1990, 20, 4509). Furthermore, NPY sequences from a number of animal species have been elucidated and all show a high degree of amino acid homology to the human protein (more than 94% in rat, dog, rabbit, pig, cow, sheep) (see Larhammar, D. in “The Biology of Neuropeptide Y and Related Peptides”, Colmers, W. F. and Wahlestedt, C. Eds., Humana Press, Totowa, N.J. 1993).
Endogenous receptor proteins that bind NPY and related peptides as ligands have been identified and distinguished, and several such proteins have been cloned and expressed. Six different receptor subtypes [Y1, Y2, Y3, Y4(PP), Y5, Y6 (formerly designated as a Y5 receptor)] are recognized today based upon binding profile, pharmacology, and/or composition if identity is known (Wahlestedt, C. et al. Ann. N.Y. Acad. Sci. 1990, 611, 7; Larhammar, D. et al. J. Biol. Chem. 1992, 267, 10935; Wahlestedt, C. et al. Regul. Pept. 1986, 13, 307; Fuhlendorff, J. U. et al. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 182; Grundemar, L. et al. J. Pharmacol. Exp. Ther. 1991, 258, 633; Laburthe, M. et al. Endocrinology 1986, 118, 1910; Castan, I. et al. Endocrinology 1992, 131, 1970; Gerald, C. et al. Nature 1996, 382, 168; Weinberg, D. H. et al. J. Biol. Chem. 1996, 271, 16435; Gehlert, D. et al. Curr. Pharm. Des. 1995, 1, 295; Lundberg, J. M. et al. Trends in Pharmacological Sciences 1996, 17, 301). Most and perhaps all NPY receptor proteins belong to the family of so-called G-protein coupled receptors (GPCRs). The neuropeptide Y5 receptor, a putative GPCR, is negatively coupled to cellular cyclic adenosine monophosphate (cAMP) levels via the action of adenylate cyclase (Gerald, C. et al. Nature 1996, 382, 168; Gerald, C. et al. PCT WO 96/16542). For example, NPY inhibits forskolin-stimulated cAMP production/levels in a neuroblastoma cell line. A Y5 ligand that mimics NPY in this fashion is an agonist whereas one that competitively reverses the NPY inhibition of forskolin-stimulated cAMP production is an antagonist.
The neuropeptide Y2 receptor has high affinity for NPY and PYY, but unlike the Y1 receptor, is relatively resistant to the effect of the N-terminal deletion and retains a high binding affinity for the C-terminal fragments such as NPY13-36 (Blomqvist, A. G. et al. Trends Neurosci. 1997, 20, 294-298).
NPY itself is the archetypal substrate for the NPY receptors and its binding can elicit a variety of pharmacological and biological effects in vitro and in vivo. When administered to the brain of live animals (intracerebroventricularly (icv) or into the amygdala), NPY produces anxiolytic effects in established animal models of anxiety such as the elevated plus-maze, Vogel punished drinking, and Geller-Seifter's bar-pressing conflict paradigms (Heilig, M. et al. Psychopharmacology 1989, 98, 524; Heilig, M. et al. Regul. Pept. 1992, 41, 61; Heilig, M. et al. Neuropsychopharmacology 1993, 8, 357). Thus, compounds that mimic NPY are postulated to be useful for the treatment of anxiolytic disorders.
The immunoreactivity of NPY is notably decreased in the cerebrospinal fluid of patients with major depression and those of suicide victims (Widdowson, P. S. et al. J. Neurochem. 1992, 59, 73), and rats treated with tricyclic antidepressants display significant increases of NPY relative to a control group (Heilig, M. et al. Eur. J. Pharmacol. 1988, 147, 465). These findings suggest that an inadequate NPY response may play a role in some depressive illnesses, and that compounds that regulate the NPY-ergic system may be useful for the treatment of depression.
NPY improves memory and performance scores in animal models of learning (Flood, J. F. et al. Brain Res. 1987, 421, 280) and therefore may serve as a cognition enhancer for the treatment of neurodegenerative diseases such as Alzheimer's Disease (AD) as well as AIDS-related and senile dementia.
Elevated plasma levels of NPY are present in animals and humans experiencing episodes of high sympathetic nerve activity such as surgery, newborn delivery and hemorrhage (Morris, M. J. et. al. J. Auton. Nerv. Syst. 1986, 17, 143). Thus, chemical substances that alter the NPY-ergic system may be useful for alleviating migraine, pain, and the condition of stress.
NPY also mediates endocrine functions such as the release of luteinizing hormone (LH) in rodents (Kalra, S. P. et. al. Front. Neuroendrocrinol. 1992, 13, 1). Since LH is vital for mammalian ovulation, a compound that mimics the action of NPY could be useful for the treatment of infertility, particularly in women with so-called luteal phase defects.
NPY is a powerful stimulant of food intake; as little as one-billionth of a gram, when injected directly into the CNS, causes satiated rats to overeat (Clark, J. T. et al. Endocrinology 1984, 115, 427; Levine, A. S. et al. Peptides 1984, 5, 1025; Stanley, B. G. et al. Life Sci. 1984, 35, 2635; Stanley, B. G. et al. Proc. Nat. Acad. Sci. U.S.A. 1985, 82, 3940). Thus NPY is orexigenic in rodents but not anxiogenic when given intracerebroventricularly and so antagonism of neuropeptide receptors may be useful for the treatment of diabetes and eating disorders such as obesity, anorexia nervosa, and bulimia nervosa.
It is known that the anxiolytic properties of NPY are mediated through postsynaptic Y1 receptors, whereas presynaptic Y2 receptors negatively control the release of NPY and other cotransmitters (e.g. GABA). Consequently, antagonism of the Y2 receptor may lead to enhanced GABAergic and NPYergic effects and Y2 receptor antagonists should prove useful in the treatment of depression and anxiety.
Recently, a key role of presynaptic hypothalamic Y2 receptor has been suggested in central coordination of energy homeostasis and bone mass regulation (Herzog, H. et al. Drug News & Perspectives 2002, 15, 506-510). Studies analyzing Y2 receptor knockout mice have started to unravel some of the individual functions of this receptor subtype. Y2 receptor knockout mice do show a reduced body weight despite an increase in food intake, which is possibly due to the lack of the feedback inhibition of the postprandially released PYY3-36 (Batterham, R. L. et al. Nature 2002, 418, 650-654). The Y2 receptor knockout mice also show a significant increase in bone formation (Baldock, P. A. J. Clin. Invest. 2002, 109, 915-921). Specific deletion of the Y2 receptor in the hypothalamus in adult conditional Y2 receptor knockout mice also causes an increase in bone formation.
Grouzmann and coworkers described a peptide-based ligand, T4-[NPY 33-36], which showed considerable affinity (IC50=67 nM) for the NPY Y2 receptor (Grouzmann, E., et al. J. Biol. Chem. 1997, 272, 7699-7706). BIIE0246 also binds to the NYP Y2 receptor with significant affinity (IC50=3.3 nM) (Doods, H., et al. Eur. J. Pharmacol. 1999, 384, R3-R5). However, the therapeutic potential for these compounds is limited due to their peptide-like composition and elevated molecular weight.
Studies also indicate that NPY Y2 is involved in the neurobiological responses to ethanol and other drugs of abuse. Thiele and coworkers (Neuropeptides, 2004, 38(4), 235-243; Peptides 2004, 25(6), 975-983) described the low ethanol consumption of Y2 receptor knockout mice, as well as their increased voluntary water consumption. Therefore, modulators of NPY Y2 may allow for the treatment of alcohol and drug abuse.
Accordingly, it is an object of the present invention to provide novel non-peptidic NPY Y2 receptor inhibitors that are useful in modulating or treating: anxiolytic disorders and depression; a condition requiring treatment of injured mammalian nerve tissue; a condition amenable to treatment through administration of a neurotrophic factor; a neurological disorder; bone loss; substance related disorders; obesity; an obesity-related disorder; and a condition related to an endocrine function including inovulation and infertility.